In the US, Bank of America, Citigroup, Goldman Sachs and JP Morgan are among those marked down.

BBC business editor Robert Peston reported on Tuesday that the downgrades were coming and said that banks were concerned as it may make it harder for them to borrow money commercially.

"All of the banks affected by today's actions have significant exposure to the volatility and risk of outsized losses inherent to capital markets activities," Moody's global banking managing director Greg Bauer said in the agency's statement.

Moody's said it recognised, "the clear intent of governments around the world to reduce support for creditors", but added that they had not yet put the frameworks in place that would allow them to let banks fail.

Some of the banks were put on negative outlook, which is a warning that they could be downgraded again later, on the basis that governments may eventually manage to withdraw their support.

“Start Quote

The most interesting thing about the Moody's analysis is that it, in effect, creates three new categories of global banks, the banking equivalent of the Premier League, the Championship and League One”

In a statement, RBS responded to its downgrade saying: "The group disagrees with Moody's ratings change which the group feels is backward-looking and does not give adequate credit for the substantial improvements the group has made to its balance sheet, funding and risk profile."

The BBC's Scotland business editor Douglas Fraser tweeted: "Cost of RBS downgrade by Moody's: having to post an estimated extra £9bn in collateral for its debts."

Of the banks downgraded, four were cut by one notch on Moody's ranking scale, 10 by two notches and one, Credit Suisse, by three notches.

"The biggest surprise is the three-notch downgrade of Credit Suisse, which no one was looking for," said Mark Grant, managing director of Southwest Securities.